Chlorine and caustic soda are produced commercially by electrolytic methods, primarily from aqueous solutions of alkali metal chlorides. In the electrolysis of brines, chlorine is produced at the anode and hydrogen, together with an alkali metal hydroxide, such as sodium or potassium hydroxide at the cathode. As the anode and cathode products must be kept separate many cell designs have been developed utilizing a separator, such as a diaphragm or membrane or a mercury intermediate electrode to separate anolyte and catholyte products.
In the diaphragm cell process, brine is fed continuously into the electrolytic cell and flows from the anode compartment through an asbestos diaphragm into the cathode compartment. To minimize back-diffusion and migration, the flow rate is adjusted such that only part of the salt is converted. The hydrogen ions are discharged from the solution at the cathode forming hydrogen gas and leaving hydroxyl ions. This catholyte solution, containing sodium hydroxide and unchanged sodium chloride, is evaporated to obtain sodium hydroxide product. In the course of the evaporation the sodium chloride precipitates, is separated, redissolved, and recirculated back into the cell. The function of the diaphragm, conventionally asbestos or polymer reinforced asbestos, is to maintain the level of concentration of alkali to minimize diffusional migration of hydroxyl ions into the anolyte, and to maintain separation of hydrogen and chlorine. Of course, it is desirable that the diaphragm also have minimal electrical resistance and extended life in the environment of the cell.
In the mercury cell process, an alloy or amalgam is formed between the discharged cation and the mercury. This amalgam flows or is pumped to a separate reaction chamber where it is allowed to undergo reaction, most often with water to form hydrogen and a comparatively strong sodium hydroxide solution containing almost no sodium chloride.
The diaphragm process is inherently cheaper than the mercury process, but as the former process does not provide chloride-free alkali additional processing steps are necessary to purify and concentrate the alkali.
Substitution of an ion exchange membrane material for the diaphragm has been proposed. Numerous permselective liquid impermeable membrane materials have been suggested. For example, membranes are described in U.S. Pat. Nos. 2,636,851; 2,967,807; 3,017,338; and British Pat. Nos. 1,184,321 and 1,199,952. Such membranes are substantially impervious to hydraulic flow. During operation, brine is introduced into the anode compartment wherein chlorine is liberated. Then, in the case of cation permselective membrane, sodium ions are transported across the membrane and into the catholyte compartment by ion exchange mechanism. The concentration of the relatively pure caustic produced in the catholyte compartment is determined by the amount of water added to this compartment from an external source, as well as by the migration of water in the cell, i.e. . . . by osmosis and/or electro-osmosis. While operation of a membrane cell has many theoretical advantages, its commercial application to the production of chlorine and caustic has been hindered by the often erratic operating characteristics of the cells. A number of disadvantages have been present when using these membranes, including a relatively high electrical resistance, poor permselectivity and oxidative degeneration, as well as relatively high cost.
As an alternative to asbestos diaphragms and liquid impermeable ion exchange membranes, industry has sought a suitable porous or microporous polymeric diaphragm material. Such a diaphragm material would for example have such desirable characteristics as maximum chemical stability, low electrical resistance, and hydraulic properties similar to an asbestos diaphragm. Other necessary properties include sufficient mechanical strength to withstand handling during assembly of a cell, shape and dimensional stability when wet with electrolyte, controlled porosity and sufficient density to act as a physical barrier to resist pentration of the matrix by gaseous reactants. That is, the polymeric diaphragm matrix must have a porosity sufficient to permit sufficient flow of brine from the anolyte to the catholyte compartment to maintain a desired caustic concentration, minimize caustic back migration and separate gaseous chlorine and hydrogen at minimum electrical resistivity, and be inert to the electrolyte system.
References may be found relating to such microporous diaphragm materials. Mention may be made more particularly to the following patents which employ techniques of compression pore forming followed by fritting or sintering, or techniques of coagulation of a mixture for the deposition on a support.
French Pat. No. 1,491,033, of Aug. 31, 1966, relates to a process for manufacturing porous diaphragms which consists of mixing a solid additive in particulate form into an aqueous dispersion of polytetrafluoroethylene in the presence of particulate inorganic fillers, coagulating the dispersion, placing the resultant coagulum in sheet form, and removing the solid particulate additive from the sheet. The removable particulate additive generally consists of starch or calcium carbonate, and is removable by immersion of the resultant sheet in hydrochloric acid. Alternatively, the additive may also be a polymer which is soluble in an organic solvent, or depolymerizable, or evaporable upon heating of the sheet. The particulate inorganic fillers which are suitable include barium sulfate, titanium dioxide, and asbestos.
U.S. Pat. No. 3,890,417, issued June 17, 1975, teaches a method for manufacturing a porous diaphragm comprising preparing an aqueous slurry or dispersion consisting of polytetrafluoroethylene and solid particulate additive, thickening and aqueous slurry or dispersion to affect agglomeration of solid particles therein, forming a dough-like material containing sufficient water to serve as a lubricant in a subsequent sheet forming operation, forming a sheet of desired thickness, and removing the solid particulate additive from the sheet. The solid particulate additive may be any which is substantially insoluble in water, but which is removable by a suitable chemical or physical means. Examples indicated are starch, and calcium carbonate.
U.S. Pat. No. 3,281,511, issued Oct. 25, 1956, discloses preparing microporous polytetrafluoroethylene resin sheets by mixing fine polytetrafluoroethylene powder with a carrier and readily removable filler material, rolling the thus made dough with intermediate reorientation, so that the particles are biaxially oriented. The solvent is then evaporated and the polytetrafluoroethylene is sintered at above its melting temperature, followed by removal of the filler by an appropriate solvent. The carrier material is readily vaporizable material such as naptha, or petroleum distillate, such as Stoddard solvent, which is a standard petroleum distillate having a flash point not lower than 100.degree. F., comprised largely of saturated hydrocarbons.
U.S. Pat. No. 3,556,161, issued Jan. 19, 1971, relates to polytetrafluoroethylene sheet materials formed by the "fritforming" process, comprising mixing polytetrafluoroethylene powder with a liquid such as kerosene, and then sequentially working the resultant composition by the application of concurrent compressive stress and shear stress, the sequence of operation being directed so the shear stress components are distributed substantially biaxially, resulting in planar orientation in the resultant article. As is the case with the materials formed by the process of U.S. Pat. No. 3,281,511, the sheet material is biaxially oriented and of high tensile strength.
British Pat. No. 1,473,286, discusses diaphragms fabricated from PTFE and a pore forming agent. As aqueous homogeneous paste is rolled to form the diaphragm, followed by sintering and removal of the pore forming agent. The actual current efficiencies of such diaphragms during electrolysis are below commercially acceptable levels.
These and other well known techniques, in the case of polytetrafluoroethylene, have not been capable of producing microporous diaphragms having acceptable performance, in that satisfactory mechanical properties, such as proper porosity etc. have not been achieved.
It is an object of the present invention to provide microporous separators suitable for electrolytic cells. It is further an object of this invention to identify those necessary characteristics of a microporous diaphragm, to achieve the necessary balance of properties for a commercially acceptable electrolytic cell separator.